The invention relates more specifically to a so called rotary drum filter, the basic construction of which comprises a tank, containing the suspension being thickened and a wire-surfaced cylinder rotatably mounted in the tank. The ends of the drum or cylinder are sealed so as to prevent the cylinder from being filled with suspension. The cylinder has under its wire surface filtrate compartments, into which liquid from the suspension in the tank is pressed due to the hydrostatic pressure, whereby the fibers in the suspension are transferred with the liquid onto the wire surface and are thickened forming a fiber mat thereon. The cylinder in this kind of filter based on hydrostatic pressure can be submerged in the suspension in the tank only slightly below the axial plane of the cylinder. When the cylinder rotates slowly, approximately 1 to 3 rpm, a fiber mat is gradually deposited on the wire surface. The formation speed of the mat naturally depends not only on the thickness of the deposited mat, but also on how deep in the vessel the filtration takes place. The filtration reaches its maximum speed slightly before the bottom dead center, because, at this point, the hydrostatic pressure is close to its maximum and the build-up of a thick mat does not yet disturb the filtration. After the bottom dead center, the formation of the mat slows down, until the formation, of course, ceases completely, when the mat emerges from the suspension.
As the fiber mat emerges from the tank upon rotation of the cylinder the filtration based on gravity begins, liquid flows slowly due to the effect of the gravity through the fiber mat into the filtrate compartments inside the wire surface. The fiber mat is removed from the cylinder at a stage, when it has passed the top dead center of the cylinder and again approaches the surface of the suspension. The mat may be removed either mechanically with scrapers or by injecting either a liquid or compressed air through the wire surface. The liquid which has flowed into the filtrate compartments may be discharged from the apparatus, for example, via the hollow shaft of the apparatus or by utilizing some other arrangement. The length of a filtration cycle based on hydrostatic pressure in an apparatus according to the above description is approximately 140.degree. of a revolution of a cylinder and the proportion of the initial filtration is approximately 30.degree. and the proportion of the vacuum drying (above the liquid surface of the tank) is approximately 100.degree..
The above-described basic type of a drum filter has been considerably improved over the decades. One significant improvement worth mentioning is the subjecting of the suspension in the tank to a suction applied through the compartments, whereby the filtration of liquid is considerably improved and thus the capacity of the apparatus is significantly increased. Generally, the suction operation is carried out by a suction leg in such a way that each of the filtrate compartments is connected by means of a separate filtrate tube to a valve which is connected to the suction leg and the shaft of the apparatus. The operation of the valve is controlled in such a way that the fiber mat is subjected to suction over a sector of approximately 240.degree.. Approximately 30 filtrate compartments are arranged within the cylinder, each of which compartments has been connected by a separate filtrate tube to the shaft tube of the cylinder surrounding the valve. Since the fiber mat generated this way is significantly more tightly attached to the filtrate surface than the mat generated in a conventional way, the thickened fiber mat must be removed from the filtrate surface by separate means. At this point, it should be noted that the suction leg may be replaced by some other apparatus creating subatmospheric, such as a vacuum pump.
The filtrate compartments in accordance with the prior art have one disadvantage, which occurs particularly in the operation stage of the filter cylinder, when the pulp web is removed from the wire surface of the cylinder. At this point the filter cylinder described in, for example, FI Patent 55538 and the filter compartments therewherein have turned, in the mat removal stage to a position wherein the filtrate still present in the compartments, will flow, in the rotational direction of the cylinder, toward the front wall as it is unable to exit from the compartment, since the discharge openings of the compartment do not usually extend along the entire width of the bottom of the compartment. Consequently, especially when the suction is interrupted and the compartment turns further, the filtrate begins to flow against the wire surface, whereby it, of course, permeates the wire surface and flows back to the tank or to the pulp web to dilute the consistency thereof.